The gauge-figure task (GFT) is a widespread method used to study surface perception for evaluating rendering and visualization techniques. The authors investigate how accurately slant angles probed on well-defined objects align with the ground truth (GT) in monoscopic and stereoscopic displays. Their results show that the GFT probes taken with well-defined objects align well with the GT in the all-monoscopic and all-stereoscopic conditions. However, they found that a GF rendered in stereo over a monoscopic stimulus results in a strong slant underestimation and that an overestimation occurred in the inverse case (monoscopic GF andstereoscopic stimulus). They discuss how their findings affect the interpretation of absolute GFT measures, compared to the GT normal.